Nearly two centuries ago, Jenner used a live virus of another species to combat smallpox—one of the most lethal human pathogens known. In the intervening years, science has provided the tools to produce by design in the laboratory other live viruses capable of protecting against their more lethal siblings. We have learned to attenuate human pathogenic viruses by passage in nonhuman hosts, by cultivation at lower temperature, and by the genetic engineering of mutations in viral genomes. Science has not yet ablated the misery of human infectious disease. Indeed, as measured in terms of health costs, human diseases caused by human immunodeficiency virus, influenza, and the herpesviruses account for a very significant portion of the total costs. While efforts designed to eliminate other infectious diseases from human society continue, other uses for viruses emerged. They stem from four considerations.